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Researchers in Sweden have found a slight reduction in blood flow rate in ophthalmic arteries (OA) of normal-tension glaucoma (NTG) patients, indicating that blood flow may play an important role in the development of the disease.
How NTG develops isn’t fully understood, according to researchers. They point to previous studies that have indicated that the disease’s pathophysiology differs from that of high-tension glaucoma, and implicate vascular issues as a major factor. Links between NTG and risk factors such as low systemic blood pressure, vascular dysregulation, vasospasm, migraines, and gender have been suggested.
The common method for assessing flow in retrobulbar vessels is color Doppler imaging; however, this technique does not measure volumetric blood flow rate. Phase-contrast magnetic resonance imaging (PC-MRI) can measure blood flow rate and, researchers say, has demonstrated an ability to quantify blood flow rate in small vessels, such as the OA.
To investigate the feasibility of using PC-MRI to assess and characterize the blood flow rate of OA in NTG patients in comparison with healthy controls, researchers recruited patients with uni- or bilateral NTG with optic nerve head damage and visual field defects. Subjects with neurological disease, previous intracranial surgery, coronary heart disease, and previous ocular surgery in the examined eye (except for cataract extraction) were excluded.
In total, 16 patients (10 female) with treated NTG and 16 age- and sex-matched healthy controls underwent PC-MRI using a 3-Tesla scanner and ophthalmological examinations. The findings show a trend toward a lower blood flow rate in patients with NTG, although not significant. Additionally, there was a tendency toward a reduction in blood flow rate of the worse eye compared with the better eye in the NTG patients (P =.14). The blood flow rate was correlated between the right and left eye in the same person (Pearson correlation 0.71) in NTG subjects. An interobserver analysis between 3 masked researchers shows a high intra-class correlation of 0.92 (95% CI 0.84–0.96, P <.01).
More specifically, the blood flow rate in the NTG group was approximately 9.6 ml/min compared with 11.9 ml/min in the control group. Resistance index (RI) and pulsatility index (PI) were approximately 0.73 and 1.36, respectively, in the NTG group and 0.68 and 1.22, respectively, in the healthy group. The mean visual field index (VFI) was approximately 46% for the worse NTG eyes. The measured differences observed between the NTG group and the control group in blood flow rate (P =.12), RI (P =.18), and PI (P =.27) were non-significant.
While the lower blood flow rate in NTG patients didn’t reach statistical significance, investigators contend that any reduction in the blood flow rate of NTG patients should be related to compromised blood flow to the choroidal and retinal structures. To detect a difference of 2 ml/min to 3 ml/min between the groups as indicated in this study, approximately 60 subjects in each group would be required.
The investigators acknowledge a few limitations of the study, including that the NTG group was receiving IOP-lowering treatment with eye drops. However, they note that there was no difference between the groups in either IOP or blood pressure at the time of examination. Second, a potential limitation of PCMRI is partial volume errors, which may lead to overestimated blood flow rates. To minimize this, the research team maximized spatial resolution.
The researchers also explain that, while the 3-Tesla PC-MRI methodology is likely a feasible method by which to answer if blood flow rate is reduced in NTG patients, an alternative approach would be to utilize the more powerful 7-Tesla PC-MRI technology for measurements of the central retinal and posterior ciliary arteries.
Disclosure: Several study authors declared affiliations with the biotech and pharmaceutical industries. Please see the original reference for a full list of authors’ disclosures.
Reference
Kristiansen M, Lindén C, Qvarlander S, et al. Feasibility of MRI to assess differences in ophthalmic artery blood flow rate in normal tension glaucoma and healthy controls. Acta Ophthalmol. Published online November 19, 2020. doi: 10.1111/aos.14673.
